1. Field of the Invention
The present invention relates to a machine tool having at least one working spindle, in which working spindle a tool receptacle for tool holders is provided, and having at least one tool-changing arm with a gripper, in which gripper a tool holder is held in a peripheral alignment with respect to the gripper in order to transfer this tool holder between its magazine position, which is spaced apart from the tool receptacle, and its working position, in which the tool holder can be clamped into the tool receptacle.
2. Related Prior Art
A machine tool of the abovementioned type has frequently been disclosed in the prior art.
DE 33 20 851 A1 and DE 40 31 997 A1, to the disclosure of which reference is expressly made here, disclose a machine tool of the travelling-column type, in which a working spindle having a tool receptacle, which is provided on the front side of the spindle and is intended for clamping standardized tool holders in, can be moved in three directions in space in relation to a work piece. The working spindle is mounted rotatably in a spindle housing on which a sleeve is arranged in a vertically displaceable manner, the sleeve bearing a plurality of tool-changing arms distributed around the working spindle. Each tool-changing arm bears at its lower, free end a gripper, in which a tool holder is held, which can be clamped into the tool receptacle and bears a machining tool on its lower shank.
The tool holders are inserted, depending on requirements, into the tool receptacle, for which purpose the tool-changing arm together with the relevant tool holder is pivoted downwards under the tool receptacle. Raising the sleeve causes the tool holder to then pass with its standardized taper, for example a steep taper or hollow tapered shank, into the tool receptacle where it is clamped in place. The gripper remains on the tool holder during the use of the tool, which is now driven by the working spindle, as a result of which a rapid tool change is possible.
In other words, the tool-changing arms arranged around the working spindle use their grippers to keep the various tool holders in their magazine position, in which they are spaced apart laterally and upwards with respect to the tool receptacle. One tool in each case is pivoted into the working position and, by the sleeve being moved, is inserted into the tool receptacle. While the tool-changing arm with the tool which was previously in operation is being pivoted upwards, another tool-changing arm can already be pivoted downwards in order to clamp a new machining tool into the tool receptacle via the relevant tool holder.
In many cases, it is important to know, in the case of a machine tool of this type, in which orientation the tool has been clamped into the tool receptacle, this being of importance, for example, in the case of a rearwards core drill.
The patent literature describes various constructions for how tools can be inserted “in a positionally correct manner”, i.e. in a defined manner with respect to their peripheral alignment, into the tool receptacle of a working spindle.
For example, EP 0 354 467 A2 describes a gripper for a steep-taper tool holder. A thickened, encircling collar is provided between the steep taper, which is inserted into the tool receptacle, and the shank, to which the machining tool is fastened, with a gripper groove which points radially outwards and in which the gripper engages by means of its jaws being provided in the collar. Provided on the gripper is a spring-loaded latching lever, the free end of which engages as a latching lug in a cut-out on the circumference of the tool holder, so that the tool holder is secured in a nonrotatable manner on the gripper during its transfer between the magazine position and the working position. When the tool holder is inserted into the tool receptacle, the latching lever is disengaged from the tool holder via a stop, thus enabling the tool receptacle to be inserted into the rotating spindle. In order to load the grippers, the latching lever may also be disengaged manually.
EP 0 297 828 A1 shows a comparable construction. This publication discloses a gripper which has two moveable jaws for grasping tool holders. The jaws are coupled via a deflecting gear to a bolt which is moved against a spatially fixed stop in order to open the jaws. A projection is provided on one of the jaws and, in the closed state of the jaws, engages in a cut-out on the tool holder, which also brings about here the nonrotatable securing of the tool holder on the gripper.
EP 0 761 384 A1 also shows a similar gripper construction, but the jaws here are not spread via a spatially fixed stop, but rather via a driving unit provided on the gripper, in order to release the tool holder for rotational purposes.
So that the tool holders are clamped into the tool receptacle in a defined angular position, catch slots which are of differing depth or breadth or are arranged on different radii are provided on the standardized tool holders of the steep-taper or hollow tapered shank type. Sliding blocks are provided in the working spindle and, when the tool holder and working spindle are correctly aligned peripherally, engage in the catch slots. An offset of 180° can be detected here, for example, depending on the depth of engagement of the sliding block in the catch slot, as is disclosed, for example, in EP 0 681 880 A1.
If a tool holder is inserted with the aid of the above-described grippers into the tool receptacle in a defined angular position to the working spindle, then the sliding blocks find the catch slots within a very short time, which permits a very rapid tool change. Since the tools are inserted into the tool receptacle with the working spindle rotating at a low coupling speed, the release of the tool holders for rotation in the gripper takes place before the tool holder is clamped into the tool receptacle.
EP 0 249 898 A1 discloses a gripper in which no orientation means are provided in order to hold the tool holder in a defined peripheral alignment with respect to the gripper. The tool holder rests by means of an upper tapered surface of its gripper groove on an inwardly projecting web on the jaws of the gripper, so that a type of frictional connection is produced. When the tool holder is inserted into the tool receptacle, the tool holder is rotated relative to the gripper by the sliding blocks, which rub on an annular surface in which the catch slots are provided, but the friction between the gripper and the tool holder ensures that the sliding block “migrates along” the end surface until it has reached the catch slot and engages therein. Only then is the tool holder completely clamped in the tool receptacle. So that the gripper can remain on the tool holder even during operation, the gripper is now lowered downwards somewhat, so that an air gap is formed between the gripper groove and the web and the tool holder can rotate without friction in the gripper.
A drawback of the last-mentioned construction is that the tool-changing time is not defined since the tool holder is set in any desired angular position of the working spindle, which means that the search run of the sliding blocks can last for different lengths of time. In addition, depending on the level of friction between the tool holder and gripper, on the one hand, and tool holder and sliding block, on the other hand, it may take a disproportionately long amount of time until the sliding block has found the correct catch slot.
In order to shorten the tool-changing time and to ensure a defined peripheral alignment of the tool holder with respect to the working spindle, the above-discussed grippers are therefore frequently used with the orientation means, with it being a drawback there that the grippers are of mechanically complex construction.